Very slow M4 transitions and shell-model intruder states in 199, 201Bi

Abstract

The isomeric decays of the 1 2 + shell-model intruder states in 199,201Bi have been investigated. In the case of 201Bi we show that the s 1 2 → h 9 2 isomeric transition is pure M4 within experimental error and is 2200 times slower than any other M4 transition observed in odd-mass nuclei. Its l-forbiddenness and the possibility of an E5 admixture and penetration effects are discussed.